In the prior art, sprayed coatings having high corrosion resistance are used for protecting substrates in a variety of service environments. While aluminum, chromium and similar metal oxides are often used as the thermal spray material, the sprayed coatings of these oxide materials are susceptible to corrosion upon exposure to hot plasma. These materials are thus inadequate for use in the semiconductor manufacturing process which may typically involve treatment in a halogen-based corrosive gas plasma atmosphere.
The halogen-based corrosive gas plasma atmosphere used in the fabrication of semiconductor devices contains fluorine-based gases such as SF6, CF4, CHF3, ClF3 and HF or chlorine-based gases such as Cl2, BCl3 and HCl.
Known articles which can be used in such extremely corrosive atmospheres include, for example, articles having corrosion resistant coatings formed thereon by spraying yttrium oxide (Patent Document 1) and yttrium fluoride (Patent Documents 2 and 3) to their surface. While rare earth element oxide sprayed articles are generally prepared by plasma spraying rare earth element oxide, they are long used as the sprayed articles in the industrial semiconductor fabrication process because of least technical problems. On the other hand, the rare earth element fluoride sprayed coatings suffer from a technical problem despite good corrosion resistance. The plasma spraying of rare earth element fluoride has the problem that when the rare earth element fluoride is passed through a flame at 3,000° C. or higher for melting, the fluoride can be decomposed so that the material partially converts to a mixture of rare earth element fluoride and rare earth element oxide. For this reason, practical utilization of rare earth element fluoride sprayed articles is delayed as compared with the rare earth element oxide sprayed articles.